Devices, also known as “oil separators” or “oil traps” already exist and are incorporated into the cylinder head cover of an internal combustion engine and comprise, firstly, means designed to remove the liquid oil that enters the cylinder head cover and, secondly, means designed to remove oil droplets or particles from the crank case gases.
In order to carry out this diphasic gas-oil separation, it is possible to plan on using a cyclone, which exploits the inertia of the oil droplets or particles by making them spin in a gas flow, in order to throw them, under centrifugal force, against walls along which they will flow downward, notably under the effect of gravity, in order ultimately to be recovered and removed.
As FIG. 1 of the attached schematic drawing shows, a conventional cyclone designed to separate a liquid phase from a gaseous phase comprises, from top to bottom:                an upper tangential inlet 1 for the gases that contain droplets or liquid particles that are to be removed,        a cylindrical zone 2 for collecting the droplets or particles,        a conical zone 3 for recovering the droplets or particles,        a lower zone 4 for discharging and/or storing the separated-off liquid phase.        
The cyclone also usually has an upper opening 5 at its top acting as an axial outlet for the gases, which have been separated from the particles or droplets they used to contain, the path 6 of the gases through the cyclone being first of all downward and helical, then upward and tending toward an axial direction.
The effectiveness of such a cyclone is connected to the speed of the gaseous flow passing through it: the higher the speed of the flow, the greater the inertia possessed by the droplets or liquid particles which will therefore be thrown more violently and more reliably against the walls, particularly the cylindrical wall of the collection zone.
Thus, in order to achieve maximum effectiveness, the speed of the gaseous flow within the cyclone ought to be as high as possible. However, in the application under consideration here, the speed of the gaseous flow through the cyclone is not constant but is dependent on the flow rate of crank case gases produced by the internal combustion engine, this gas flow rate itself varying as a function of engine speed and load. In particular, under part-load conditions, the flow rate of crank case gases is lower than it is under full load. Likewise, for low engine speeds, the speeds of the gases are lower, and the effectiveness of the cyclone is therefore lower. It may thus be considered that high cyclone efficiency corresponds to high gas flow velocities and that low cyclone efficiency corresponds to low gas flow velocities, at least if the geometric characteristics of the cyclone remain unchanged.
Simply increasing the speed of the flow, particularly for low flow rates, would not provide a satisfactory solution to the problem addressed here because by setting a high speed for low flow rates, notably by reducing the dimensions of the cyclone, a pressure drop would be created that would prove too great for high flow rates.
Conversely, sizing the cyclone in order to limit the pressure drop at high flow rates would yield a cyclone that was even less effective at low flow rates.
One other potential solution might be to vary the cross section of the upper gas inlet to the cyclone, reducing this cross section when the gas flow rate is low, and increasing this cross section when the gas flow rate is high, so that the gases always enter the cyclone at the same speed. That could provide a certain degree of improvement, if it is reckoned that the speed of the gases on entering the cyclone is maintained over part of the path of these gases through the cyclone. However, the sudden variations in cross section would in this case create pressure drops which would restrict the initial energy of the jet of gas.
In order to find a suitable solution to the problem addressed here, and therefore in order to maintain constant and, if possible, high cyclone efficiency for any incoming gas flow rate, it would therefore appear preferable not to alter the gas inlet conditions, or the inlet alone, but to impose a constant speed on the flow actually inside the cyclone, using suitable regulation measures.